This invention is in the field of Digital Communications; it relates particularly to the translation of information from one digital form to another.
As digital communications proliferate, and more and more information is communicated via digital facilities, the need to change codes and formats to fit the facility also increases. In one notable example, a system for inserting broadband audio program signals into T carrier time slots, which is described in the Gaunt-Giammusso application referred to above, encodes samples of the program signal into 14-bit linear pulse code modulated (PCM) wordsin offset-binary form. The 14-bit words are then compressed into 11-bit PCM words in sign-magnitude form, the order of the bits is rearranged, and a parity bit is added to reflect parity over only certain, not all of the bits. In another example, the Federal Republic of Germany has proposed to CCITT in Contribution #109 to Study Group XV, May, 1982, another format for compressed 12-bit words for sound program signals. According to the German proposal, the 12-bit words are also made up of eleven PCM compressed code bits and one parity bit; the more significant bits are interleaved with the less significant bits, and parity is taken over only the five most significant bits.
While 8-bit compressed analog-to-digital encoders are common, 11-bit compressing encoders are not. Furthermore, in some cases, such as the Gaunt-Giammusso arrangement, an unusual compression characteristic is required. Fourteen or fifteen bit linear encoders, on the other hand, are easily purchased, and the resulting output words can be compressed by a digital code translator.
It is well-known in the art to translate from one code to another by means of a memory look-up table. Each possible input code word forms an address of a read-only memory (ROM). Stored at the address is the translated output code word. In the case of a 14-bit input code, however, such a scheme requires 2.sup.14 memory addresses. With 12 bits per output word, the total memory requirement would be 2.sup.14 .times.12 =196,608 bits of memory, a rather large amount. Other available translation methods involve the use of logic circuitry. For very simple translations, available logic chips can be applied, but more complicated translators require considerable design effort and often custom logic integrated circuits. Unless contemplated production is very high volume, such expenditure may not be warranted.
An object of this invention is an inexpensive digital code translator that can convert linearly encoded PCM words to compressed PCM words.
Another object is a digital code translator that can perform compression, selective bit inversion, bit order rearrangement and selective parity inclusion.
A third object is a code translator that can be assembled from readily available parts.
A fourth object is a code translator the output format of which can be entirely changed by a mere change of the contents of a memory.